1. Field of the Invention
The present invention relates to a charge trap film to be used in a nonvolatile semiconductor storage device and a producing method thereof. Furthermore, the present invention relates to a nonvolatile semiconductor storage device provided with a charge trap film and a producing method thereof.
2. Description of Conventional Art
Conventionally, as a nonvolatile semiconductor storage device in which charges are held in a charge trap film to store information, a memory cell having an MNOS (metal-nitride-oxide-semiconductor) structure or an MONOS (metal-oxide-nitride-oxide-semiconductor) structure is known.
As shown in FIG. 10, the MNOS-type memory cell has a gate structure in which a tunnel oxide film 2, a silicon nitride film 3, and a gate electrode 4 are stacked, for example, on a P-type silicon substrate 1, and drain and source diffusion layers 5 and 6 which are N.sup.+ regions are formed on both sides of the gate structure in the substrate. The MONOS-type memory cell, on the other hand, has a gate structure in which a tunnel oxide film 7, a silicon nitride film 8, a top oxide film 9, and a gate electrode 10 are stacked as shown in FIG. 11.
In those memory cells, the silicon nitride film 3 or 8 is used as a charge trap film for storing signal charges, and a positive high voltage is applied to the gate electrode 4 or 10 for writing, so that electrons tunnel from a channel region of the substrate 1 through the tunnel oxide film 2 or 7 so as to be injected into the silicon nitride film 3 or 8 to be trapped therein. Here, the silicon nitride film 3 or 8 acting as the charge trap film is formed by a CVD (chemical vapor deposition) method, and specifically, the silicon nitride film 3 or 8 is produced by making an SiH.sub.4 gas or SiH.sub.2 Cl.sub.2 gas react with an NH.sub.3 gas in an atmosphere of 700.degree.-900.degree. C.
In the conventional examples having such a structure, however, there has been a problem that in a silicon nitride film acting as a charge trap film, charges are apt to be de-trapped because the trap level is so shallow that the signal charge holding property is poor.
Then, it is proposed in Japanese Patent Application that a semiconductor storage device using, as a charge trap film, a film (so-called Si rich oxide film) containing a mixture of conductive silicon and insulating silicon dioxide. An Si rich oxide film is superior in signal charge holding property because the film has a deep level theoretically. Since the foregoing proposed Si rich oxide film is formed through a sputtering or CVD method, however, there has been a problem that it is difficult to control an insulating property between silicon crystal grains (hereinafter referred to as "silicon grains") in the oxide film. Particularly in the case where the Si rich oxide film is made thin so as to make a wiring voltage low, an inconvenient phenomenon that electrons injected in the Si rich oxide film at the time of signal charge writing pass through as they are to a gate electrode has sometimes occurred.